A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage waveform into a specified output voltage waveform. A boost power converter is one example of a switched-mode converter that converts the input voltage to an output voltage that is greater than the input voltage. Typically, the boost power converter is employed in off-line applications wherein power factor correction is required and a stable regulated voltage is desired at the output of the power converter.
A non-isolated boost power converter generally includes an energy storage device (e.g., an inductor) coupled between the input voltage and an inverter or switching device. The switching device is then coupled to a rectifier (e.g., a power diode) and an output capacitor. The load is connected in parallel to the capacitor. Again, the output voltage (measured at the load) of the boost power converter is always greater than the input voltage. When the switching device is conducting, the diode is reverse biased thereby isolating the output stage. During this period, the input voltage supplies energy to the inductor. When the switching device is not conducting, the output stage receives the energy stored in the inductor for delivery to the load coupled to the output of the converter.
Analogous to all types of power converters, a boost converter is subject to inefficiencies that impair the overall performance of the power converter. More specifically, the rectifying diode suffers from a reverse recovery condition thereby producing excessive power losses in both the rectifying diode and the switching device and oscillations in both current and voltage therefrom. The effect of the reverse recovery condition is more severe in non-isolated converters, such as the boost power converter, due to the low impedance across the input voltage source during the commutation of the rectifying diode. The reverse recovery condition can also detrimentally affect the longevity of the components, especially the rectifying diode and switching device, of the boost power converter. Therefore, efforts to minimize the losses associated with the rectifier and switching device and, more specifically, with the rectifying diode will improve the overall performance of the power converter.
A traditional manner to reduce the losses associated with the rectifying diodes is to introduce a snubber circuit coupled to the rectifying diodes. Snubber circuits are generally employed for various functions including to minimize overcurrents and overvoltages across a device during conduction and non-conduction periods and to shape the device switching waveforms such that the voltage and current associated with the device are not concurrently high. For instance, with respect to rectifying diodes, a snubber circuit may be employed to minimize oscillations in both voltage and current and power losses associated therewith due to reverse recovery current resulting from a snap-off of the rectifying diode during a transition from a conduction to non-conduction mode of operation.
Snubber circuits are well known in the prior art. One approach to reduce the reverse recovery current of the rectifying diode is to employ a snubber circuit that includes an inductor in series with the rectifying diode. This type of lossless snubber circuit attempts to recovery the energy stored in the snubber inductor during the reverse recovery period of the rectifying diode for delivery to the output of the converter. While the lossless inductor snubber provides an alternative for reducing the reverse recovery current of the rectifying diode, for reasons that will become more apparent there are tradeoffs in the selection of the inductor and auxiliary components of the lossless snubber that detract from the advantages of employing such a snubber circuit.
Accordingly, what is needed in the art is a circuit that moderates a peak reverse recovery current of a rectifier that maintains the advantages associated with lossless snubber circuits, but overcomes the contradictions presently available in the design thereof.